The invention relates generally to the field of pipe used to convey instruments along wellbores drilled through the Earth's subsurface. More particularly, the invention relates to structures for repeaters used in “wired” drill pipe systems.
Rotary drilling systems known in the art for drilling wellbores through subsurface Earth formations typically use threadedly coupled segments (“joints”) of pipe suspended at the Earth's surface by a drilling unit called a “rig.” The pipe is used, in association with certain types of tools such as drill collars and stabilizers to operate a drill bit disposed at the longitudinal end of a “string” of such pipe joints coupled end to end. As a wellbore is drilled, and it becomes necessary to lengthen the string of pipe, additional joints of pipe are coupled to the string by threading them onto the upper (Earth's surface) end of the string of pipe. Removing the string of pipe from the wellbore, such as to replace a drill bit, requires uncoupling joints or “stands” (segments consisting of two, three or four coupled joints) of the pipe string and lifting the string from the wellbore. Such coupling and uncoupling operations are an ordinary and necessary part of drilling a wellbore using a rig and such pipe strings (“drill strings”).
It is known in the art to include various types of measuring devices near the lower end of a drill string in order to measure certain physical parameters of the wellbore and the surrounding Earth formations during the drilling of the wellbore. Such instruments are configured to record signals corresponding to the measured parameters in data storage devices associated with the measuring devices. The measuring and storing devices require electrical power for their operation. Typically such power is provided by batteries and/or a turbine powered electrical generator associated with the measuring devices. The turbine may be rotated by the flow of drilling fluid (“mud”) that is pumped through a central passageway or conduit generally in the center of the pipes and tools making up the drill string. It is also known in the art to communicate certain signals representative of the measurements made by the devices in the wellbore to the Earth's surface at or close to the time of measurement by one or more forms of telemetry. One such form is extremely low frequency (“ELF”) electromagnetic telemetry. Another is modulation of the flow of mud through the drill string to cause detectable pressure and/or flow rate variations at the Earth's surface.
The foregoing power and telemetry devices have well known limitations. There has been a longstanding need in the art of wellbore drilling to provide electrical power and a relatively high bandwidth communication channel along a drill string from the bit to the Earth's surface. Various structures have been devised to provide insulated electrical conductors in association with drill pipe to provide such power and signal channels for a drill string. Such structures are generally known as “wired” drill pipe. The features of the structures that have been developed for such insulated electrical conductor channels are related to the particular requirements for pipes used for drill strings, namely, that they must be made so as to cause as little change as possible in the ordinary handling and operation of drill pipe. As will be appreciated by those skilled in the art, such handling includes repeated threaded coupling and uncoupling. Use of the pipe string during drilling will result in application to the pipe string of torsional stress, bending stress, compressional and tensional stress, as well as extreme shock and vibration. Thus, a commercially acceptable wired drill pipe must be as far as practicable transparent to the drill operator and must be operable under the types of stresses applicable to ordinary (non-wired) drill string components.
One type of wired drill pipe is described in U.S. Patent Application Publication No. 2006/0225926 filed by Madhavan et al. and assigned to the assignee of the present invention. The wired drill pipe disclosed in the '926 publication includes a conduit for retaining wires in the wall of or affixed to the wall of a joint of drill pipe, as well as electromagnetic couplings for the wires proximate the longitudinal ends of the pipe joint. The electromagnetic couplings transfer power and signal between adjacent joints of wired drill pipe.
Irrespective of the type of wired drill pipe system used, it is desirable to include one or more signal conditioner and amplification devices called “repeaters” at selected positions along the pipe string to assure adequate signal amplitude at the Earth's surface for the signals transmitted from the devices at the lower end of the drill string, and vice versa. A repeater used with wired drill pipe is typically disposed in a short-length segment (about 3 to 10 feet or 1 to 3 meters) of drill pipe or drill collar. On example of a structure for a repeater is described in U.S. Pat. No. 7,193,526 issued to Hall et al. Design challenges for signal repeaters include that the internal diameter of the device should be at least as large as the smallest internal diameter of every other component of the drill string to avoid excessive restriction on the flow of drilling fluid through the drill string; that the bending stiffness and moment of inertia are similar to that of other components of the drill string having similar outer diameter; and that the threaded connections used are essentially identical to those used in the remainder of the drill string to maintain transparency to the drill operator.
A particular issue to be addressed with repeaters in a drill string is the type of threaded connection used in typical wired drill pipe. Such connections are known as “double shoulder” connections, examples of which are described in the Madhavan et al. publication referenced above. Typically, such double shoulder threaded connections when used with wired drill pipe include a groove or similar feature formed into the internal shoulder of the threaded coupling for retaining the communication coupling. It is desirable for the repeater to have the mechanical characteristics described above, and to be usable with typical double shoulder threaded connections, to be able to transfer some of the loading applied by the inner shoulder of the adjacent threaded connection.
There continues to be a need for improvements to structures for repeaters for wired drill pipe to increase their reliability, serviceability and ease of handling during drilling operations.